Herpes viruses encode a 66 kDa protein, referred to as the maturation protease (HMP), which is essential for viral replication. C-terminal processing of the assembly protein by the protease produces active assembly protein molecules which aggregate to form the capsid scaffold. When capsid formation is complete, the protease then destroys the scaffolding, allowing entry of the viral genome into the newly formed capsid. High sequence homology of HMPs indicates that the structure of the catalytic domain of HMP will be similar among Herpes viruses. Thus, the structure of the catalytic domain of HMP from cytomegalovirus will provide a model for other HMP's in the Herpes family.